Clean and oil and then check level of switch. If possible adjust coil mounts so the movable core isn't wearing on the inside of the coil. Check for wear on insides of the coil.

Install new springs to kick the contacts off, problems disappear. Install new stationary contacts, especially carbons to add height. The sooner the contacts meet, the more spring you get for the release. But not too soon...

Your last option. You can also install a thin strip of non-magnetic metal (brass)between the cores. This last information comes from 1920's elevator books. Clean switch again.
Make sure the hinges for the movable contacts specs. include washers. One floor switch was sticking and a thin copper washer was added to the hinge to try to align the movable arm contact better (the magnetic forces tends to align the core just fine). The washer had a burr and was causing the sticking. Checked parts list, no washer, had washer removed. The floor coil operates correctly. Clean switch again.